Indicator function

A three-dimensional plot of an indicator function, shown over a square two-dimensional domain (set X): the "raised" portion overlays those two-dimensional points which are members of the "indicated" subset (X, then the indicator function of 𝟏A(x)=1 if 𝟙A and A; that is,

X, the indicator function of a subset 𝟏A:X→{0,1} defined by